Lyme disease is the most prevalent and a fast spreading vector-borne infectious disease in the U.S. It is caused by a spirochetal bacterium Borrelia burgdorferi and transmitted by the deer tick Ixodes scapularis. At least 15 genetically distinct clonal groups of the Lyme disease pathogen are circulating in the northeastern U.S., where over 80% of the Lyme disease cases are reported annually. These clonal groups differ in their wildlife prevalence and human pathogenecity. In 2006, a group of six investigators including the PI have initiated a NIH/NIAID-funded 2-year project producing the whole-genome sequences of 17 B. burgdorferi isolates, with the goal of identifying the genetic basis of clone variations in environmental invasiveness and human virulence. Here, the PI proposes a study to complete the goal of the NIAID whole-genome sequencing project by performing the comparative genome analysis of 12 most common clonal groups. Specific aims are: First, we will identify strain-specific genome changes in a phylogenetic framework. We will identify and align the main chromosomes, orthologous plasmids, and orthologous coding sequences. We will infer a genome-based phylogeny based on chromosomal DNA sequences, so that the strain differences in genome content, genome organization, and DNA sequences can be mapped to different stages during the evolutionary diversification of these clonal groups. Second, we will distinguish more consequential (e.g., adaptive) genomic changes from the lesser (e.g., random) ones by testing for the influence of natural selection. We will identify intergenic sequences important for gene regulation by their sequence conservation, genes critical for B. burgdorferi adaptation (e.g., surface lipoproteins conferring escape from host immunity) by their high non-synonymous nucleotide substitution rates relative to the synonymous rates, genes associated with initial adaptive clonal divergence by comparing the most recently diverged sister clones. Third, we will develop and maintain a website to facilitate the public dissemination of B. burgdorferi comparative genome information, such as genomic changes specific to a high-virulence clonal group. It is unknown why some strains of the Lyme disease bacteria are more pathogenic than others. We will compare the genomes of high- and low-virulence strains to identify the genes contributing to pathogenecity. Virulence-related genome elements are prime targets for designing therapeutics and vaccines.